The present invention relates to a low-thermal expansion cast steel having a high Ni content, more particularly to a low-thermal expansion cast steel having excellent machinability.
Recent development of industries in electronics and optics requires low-thermal expansion materials suffering from only small size variation due to thermal expansion and shrinkage by temperature changes near room temperature, for members constituting high-precision machine tools, high-precision measurement apparatuses, etc. To meet this demand, there have been an invar alloy of Fexe2x80x94Ni36 (by mass) having a linear thermal expansion coefficient of about 1.0xc3x9710xe2x88x926/xc2x0 C. near room temperature, and a super invar alloy of Fexe2x80x94Ni32xe2x80x94Co5 (by mass) having a linear thermal expansion coefficient of about 0.5xc3x9710xe2x88x926/xc2x0 C. near room temperature.
However, the above Fexe2x80x94Ni alloys and Fexe2x80x94Nixe2x80x94Co alloys are relatively soft, poor in machinability. Accordingly, in conventional cast products, graphite is crystallized or precipitated mainly in an austenitic matrix structure, such that graphite exhibits lubrication effects between cutting tools and the work made of low-thermal expansion materials, thereby achieving good machinability. Typical examples are cast steel ASTM A-436, TYPE 5 and A-439, TYPE D-5 containing carbon at a cast iron level (2% by mass or more) for the crystallization of graphite, and cast steel described in Japanese Patent Laid-Open No. 63-162841, in which the carbon content is increased to 0.8 weight % to precipitate graphite.
Among the above conventional low-thermal expansion materials, cast steel of ASTM A-439, TYPE D-5 has drastically improved machinability because of the precipitation or crystallization of a large amount of graphite as a machinability-improving component. However, it has as large an average linear thermal expansion coefficient as 4.0xc3x9710xe2x88x926/xc2x0 C. or more in a range of 30-100xc2x0 C. This is due to the fact that the cast steel has an increased level of micro-segregation of Ni serving to increase a thermal expansion coefficient because of the inclusion of about 2% by mass of C, and that the cast steel contains about 2% by mass of Si serving to increase a linear thermal expansion coefficient by 1.0xc3x9710xe2x88x926/xc2x0 C. per 1% by mass. In members constituting apparatuses required to have higher precision such as semiconductor production apparatuses and semiconductor test apparatuses, an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. is needed in a range of 30-100xc2x0 C., making such low-thermal expansion cast iron as ASTM A-439, TYPE D-5 unsuitable therefor.
With respect to the cast steel disclosed by Japanese Patent Laid-Open No. 63-162841, it is suitable for members requiring high precision, because its average linear thermal expansion coefficient is 2.5xc3x9710xe2x88x926/xc2x0 C. or less. However, it is much poorer in machinability than the cast iron of ASTM A-439, TYPE D-5, because the amount of graphite as a machinability-improving inclusion is only about ⅓ as compared with the cast iron of ASTM A-439, TYPE D-5.
Accordingly, an object of the present invention is to provide a low-thermal expansion cast steel having a low thermal expansion coefficient and excellent machinability.
To have an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. in a range of 30-100xc2x0 C. and machinability not lower than that of the cast iron of ASTM A-439, TYPE D-5, the amounts of C and Si should be controlled to minimize increase in a thermal expansion coefficient, thereby increasing the amounts of machinability-improving inclusions. Here, there may be two or more types of machinability-improving inclusions. The machinability-improving inclusions may be MnS, MnSe, Pb, etc. in addition to the above graphite, though Se and Pb should be avoided because of strong toxicity, causing environmental contamination.
In view of this, the inventors have found that a low-thermal expansion cast steel having excellent machinability and a suppressed linear thermal expansion coefficient in a range of room temperature to 100xc2x0 C. can be obtained by having an austenitic matrix structure in which both graphite and MnS having different functions to improve machinability are precipitated, and by minimizing the amount of elements dissolving in the matrix, which serve to increase a thermal expansion coefficient, thereby suppressing the micro-segregation of Ni. The present invention has been completed based on this finding.
Thus, the first low-thermal expansion cast steel with excellent machinability according to the present invention contains 0.3-0.9% by mass of C and 25-40% by mass of Ni, and having 0.5-3%, as an area ratio, of graphite and 0.02-0.3%, as an area ratio, of granular MnS in an austenitic matrix structure, whereby said cast steel has an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. in a range of room temperature to 100xc2x0 C.
The second low-thermal expansion cast steel with excellent machinability according to the present invention contains 0.3-0.9% by mass of C and 25-40% by mass of Ni, and having 0.5-3%, as an area ratio, of graphite, 0.02-0.3%, as an area ratio, of granular MnS, and 10-700, per 1 mm2, of plate-like MnS having a length of 8 xcexcm or more in an austenitic matrix structure, whereby said cast steel has an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. in a range of room temperature to 100xc2x0.
In a preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying Sxe2x89xa6(1/4) Mn.
In another preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05.
In a further preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.4-0.8% of C, 0.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 30-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying Sxe2x89xa6(1/4) Mn.
In a still further preferred embodiment of the present invention, the low-thermal expansion cast steel with excellent machinability has a chemical composition (by mass) comprising 0.4-0.8% of C, 0.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 30-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05.
The low-thermal expansion cast steel preferably contains 12% by mass or less, more preferably less than 4% by mass, of Co. It also preferably contains 4% by mass or less of Cr.
[1] Composition of Low-thermal Expansion Cast Steel
The low-thermal expansion cast steel of the present invention contains at least 0.3-0.9% by mass of C and 25-40% by mass of Ni.
In a preferred embodiment, the chemical composition (by mass) of the low-thermal expansion cast steel is 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying Sxe2x89xa6(1/4) Mn.
In another preferred embodiment, the chemical composition (by mass) of the low-thermal expansion cast steel is 0.3-0.9% of C, 1.5% or less of Si, 1.0% or less of Mn, 0.01-0.3% of S, 25-40% of Ni, and 0.005-0.1% of Mg, the balance being substantially Fe and inevitable impurities, the contents of S and Mn satisfying (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05.
(1) C
C has important functions of improving castability, suppressing the micro-segregation of Ni, and improving machinability by precipitation as graphite. To have enough castability, C should be 0.3% by mass or more. To precipitate graphite necessary for improving machinability, the carbon content should be 0.2% by mass or more. Also, to suppress the micro-segregation of Ni that increases a thermal expansion coefficient, the carbon content should be 0.3-0.9% by mass. Thus, the carbon content is 0.3-0.9% by mass. The preferred carbon content is 0.4-0.8% by mass.
(2) Si
Though Si is added to improve deoxidation and castability, the linear thermal expansion coefficient of the cast steel increases by about 1.0xc3x9710xe2x88x926/xc2x0 C. per 1% by mass of Si. Also, too much Si hinders the castability because of increased difference between a solidification start temperature and a solidification finish temperature. Thus, the content of Si is 1.5% by mass or less, preferably 0.5% by mass or less.
(3) Mn
Mn is added not only for deoxidation, but also for forming MnS with S to improve the machinability of the cast steel. The linear thermal expansion coefficient of the cast steel increases by about 0.7xc3x9710xe2x88x926/xc2x0 C. per 1% by mass of Mn. Thus, the content of Mn is 1.0% by mass or less, preferably 0.04-0.95% by mass, more preferably 0.3-0.95% by mass, most preferably 0.4-0.9% by mass.
(4) S
When S is added to an Fexe2x80x94Nixe2x80x94(Co) alloy containing Mn, a sulfide (MnS) with Mn is formed. MnS exists in two forms, one granular and the other plate-like. Granular MnS is formed in a range of Sxe2x89xa6(1/4) Mn, and granular MnS and plate-like MnS are formed in a range of (1/4) Mn less than S.
The granular MnS has a function of improving internal lubrication of work, resulting in decrease in shear stress necessary for generating chips, whereby cutting resistance exerted onto a cutting tool decreases. This serves to reduce the wear of a cutting tool.
The plate-like MnS has a notch function of concentrating stress, thereby generating micro cracks that propagate through the work, contributing to decrease in shear stress necessary for generating chips. As a result, cutting resistance exerted onto a cutting tool is reduced, thereby reducing the wear of a cutting tool. Further, the notch effect of plate-like MnS remarkably improves chip breakage.
To form granular MnS only, the contents of Mn and S should satisfy Sxe2x89xa6(1/4) Mn. On the other hand, to form both granular MnS and plate-like MnS, the contents of Mn and S should satisfy (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05. Because the effect of improving machinability can be obtained by the existence of both graphite and granular MnS, or the existence of graphite, granular MnS and plate-like MnS, the amount of S is determined to satisfy Sxe2x89xa6(1/4)Mn or (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05. Also, to improve machinability, S should be at least 0.01% by mass. An excess amount of S added lowers the solidification temperature in a finally solidified portion of the cast steel in a casting process, resulting in casting defects such as high-temperature cracking. Therefore, the range of S is 0.01-0.3% by mass, in addition to meeting the above equations.
When substantially only granular MnS is precipitated in an austenitic matrix structure, namely when Sxe2x89xa6(1/4) Mn is satisfied, the content of S is preferably 0.01-0.1% by mass, more preferably 0.03-0.08% by mass, most preferably 0.04-0.07% by mass. On the other hand, when substantially both granular MnS and plate-like MnS are precipitated in an austenitic matrix structure, namely when (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05 is satisfied, the content of S is preferably more than 0.1% by mass and 0.3% by mass or less, more preferably 0.11-0.25% by mass, most preferably 0.12-0.2% by mass.
(5) Ni
Ni is a main element contributing to increase in the thermal expansion coefficient of the cast steel. When an Fexe2x80x94Ni alloy is solidified, negative micro-segregation, in which a Ni concentration is lower in dendrite cores than the average Ni concentration, usually occurs. The micro-segregation of Ni locally destroys component balance for obtaining low-thermal expansion characteristics, resulting in increase in a thermal expansion coefficient. Differing from the micro-segregation of an interstitial element such as C, the micro-segregation of a substitution-type element such as Ni cannot be eliminated without diffusion annealing at 1000xc2x0 C. or higher for several tens of hours. Accordingly, to achieve a low thermal expansion coefficient under the heat treatment conditions of 800xc2x0 C. or lower, the micro-segregation of Ni should be suppressed at the time of solidification. Micro-segregation is determined by a distribution coefficient, a ratio of a solid phase to a liquid phase at solidification. If the distribution coefficient is 1, no micro-segregation occurs. In general, the distribution coefficient of Fexe2x80x94Ni alloys is about 0.8.
As a result of investigation, when the Ni content is in a range of 25-40% by mass, the distribution coefficient of Ni increases by the addition of C. At a carbon content of less than 0.3% by mass, the distribution coefficient of Ni is less than 1.0, resulting in negative micro-segregation. At a carbon content of 0.3-0.9% by mass, the distribution coefficient of Ni is almost 1.0, resulting in drastically reduced micro-segregation. When the carbon content exceeds 0.9% by mass, the distribution coefficient of Ni exceeds 1.0, resulting in positive micro-segregation, in which the Ni concentration is higher in dendrite cores than the average Ni concentration. Accordingly, the carbon content should be 0.3-0.9% by mass, to remarkably reduce the micro-segregation of Ni, thereby making it possible to keep the thermal expansion coefficient low only with graphitization annealing at 800xc2x0 C. or lower. To precipitate graphite, a machinability-improving inclusion, by the graphitization annealing, the carbon content should be at least 0.2% by mass. In view of this, at the carbon content of 0.3-0.9% by mass, micro-segregation is drastically reduced, resulting in precipitation of graphite for improving machinability.
Therefore, to achieve an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. in a range of room temperature to 100xc2x0 C., the content of Ni is 25-40% by mass, preferably 30-40% by mass.
(6) Co
Co is a further element contributing to increase in the thermal expansion coefficient of the cast steel. Though the average linear thermal expansion coefficient is less than 4.0xc3x9710xe2x88x926/xc2x0 C. in a range of room temperature to 100xc2x0 C., the existence of 12% by mass or less of Co contributes to further decrease in the thermal expansion coefficient, thereby achieving an average linear thermal expansion coefficient of less than 4.0xc3x9710xe2x88x926/xc2x0 C. more stably. Thus, the content of Co is 12% by mass or less, preferably 10% by mass or less, more preferably 9% by mass or less. Its lower limit is preferably 0.5% by mass, more preferably 1% by mass, most preferably 2% by mass. In some cases, it is less than 4% by mass.
(7) Mg
Because Mg forms MgS constituting nuclii for precipitating graphite, the content of Mg is at least 0.005% by mass. If it is contained improving too much, the crystallization and precipitation of MnS as an machinability-improving inclusion is hindered. Thus, the content of Mg should be 0.1% by mass or less.
(8) Cr
Cr is an element for increasing the thermal expansion coefficient of the cast steel without substantially changing the solidification start temperature and the solidification finish temperature. Thus, it can control the thermal expansion coefficient without deteriorating castability. If it is contained too much, different degrees of segregation of Cr occur in early-solidification portions and late-solidification portions, failing to stably control the thermal expansion coefficient of the cast steel. Thus, the Cr content is 4% by mass or less, preferably 3% by mass or less, further preferably 2.5% by mass or less. With respect to the lower limit of the Cr content, it is preferably 0.1% by mass to obtain sufficient effects thereof.
(9) Balance
The balance is substantially Fe and inevitable impurities. The amounts of inevitable impurities should be within generally accepted ranges.
[2] Structure
Because the cast steel of the present invention contains graphite and granular MnS, or graphite, granular MnS and plate-like MnS in an austenitic matrix structure, it exhibits excellent machinability with a low thermal expansion coefficient. Work made of such cast steel can be machined very easily in a reduced period of time.
(1) Graphite
In an as-cast state, the cast steel of the present invention does not contain graphite, though granular MnS or granular MnS+plate-like MnS are crystallized or precipitated in the matrix. To fully precipitate graphite for improving machinability in the matrix structure, annealing for graphitization at 550xc2x0 C. or higher is necessary regardless of heating time. When the annealing temperature for graphitization is higher than 800xc2x0 C., plate-like MnS dissolves in an austenitic matrix structure, and much smaller granular MnS than the originally crystallized granular MnS is precipitated again, resulting in drastic decrease in notch effect for improving machinability. Accordingly, to obtain both graphite and granular MnS (or granular MnS+plate-like MnS), an annealing temperature for graphitization of 550-800xc2x0 C. is optimum. To maintain both graphite and granular MnS (or granular MnS+plate-like MnS) in the structure, any heat treatment other than the annealing temperature for graphitization conducted on the cast steel of the present invention should be 800xc2x0 C. or lower.
Graphite lubricates work against a cutting tool, thereby suppressing damage on the cutting tool. The effect of improving machinability by graphite increases as the area ratio of graphite increases in the alloy structure. However, to suppress the micro-segregation of Ni, the content of C determining the amount of graphite precipitated should be 0.3-0.9% by mass. This range of carbon content provides the area ratio of graphite of 0.3-3%. Here, the area ratio of graphite is an average value determined on 50 fields of 0.2 mmxc3x970.2 mm observed by a metal microscope.
(2) MnS
Granular MnS has a function of internal lubrication during a cutting operation, thereby reducing a cutting resistance and thus suppressing the damage of a cutting tool. To obtain full effects of improving machinability by the granular MnS, the granular MnS should be present in an area ratio of at least 0.02%. On the other hand, when the area ratio of the granular MnS exceeds 0.3%, the above effects are saturated. Therefore, the area ratio of the granular MnS is 0.02-0.3%. Here, the area ratio of the granular MnS is an average value determined on 50 fields of 0.2 mmxc3x970.2 mm observed by a metal microscope. The granular MnS and graphite are easily appreciated by metal microscopic observation on a mirror-polished surface of the cast steel.
Plate-like MnS has a function of concentrating stress to exert notch effect, thereby generating micro cracks that propagate through the work, contributing to decrease in shear stress necessary for generating chips. As a result, cutting resistance exerted onto a cutting tool is reduced, thereby reducing the wear of a cutting tool. Further, the notch effect of plate-like MnS remarkably improves chip breakage.
The plate-like MnS appears like a rod or a needle in metal microscopic observation on a mirror-polished surface of the cast steel. Because MnS looking like a rod or a needle in a flat plane should be in the shape of a plate three-dimensionally, it is expressed as xe2x80x9cplate-like MnSxe2x80x9d here.
The number of plate-like MnS per 1 mm2 is an average number determined on 50 fields of 0.2 mmxc3x970.2 mm observed by a metal microscope.
If plate-like MnS is contained too much, the cast steel becomes brittle, tapping may cause various problems, such as chipping in screw threads of the work made of the low-thermal expansion cast steel. As a result of investigation, it has been found that when the number of plate-like MnS per 1 mm2 exceeds 700, the above problems occur, and that when the number of plate-like MnS per 1 mm2 is less than 10, or when plate-like MnS is as short as less than 8 xcexcm, effects of improving machinability by plate-like MnS disappear. Thus, the number of plate-like MnS should be 10-700 per 1 mm2, and plate-like MnS should be as long as 8 xcexcm or more. To obtain plate-like MnS having a length of 8 xcexcm or more in a number of 10-700 per 1 mm2, Mn and S should be in a range of (1/4) Mn less than Sxe2x89xa6(1/4) Mn+0.05.